Machines, such as wheel loaders, dozers, and other heavy equipment, are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more tires. In order to control the speed and/or torque of the tires, the operator of these machines is typically provided with one or more different foot pedals. For example, a right foot pedal can be used to affect engine fueling, while a left foot pedal can be used to affect machine braking. Additional foot pedals may be available in some applications to control connection of the engine to the transmission.
Although the typical foot pedal configuration may be suitable when a mechanical step-change transmission is utilized to transmit power from the engine to the tires, it may be insufficient when a continuously variable transmission (CVT) is utilized. A CVT is an automatic type of transmission that provides an infinite number of output ratios within its operating range. A hydraulic CVT includes a pump and a fluid motor that receives pressurized fluid from the pump. Depending on a discharge flow rate and pressure of the pump and a displacement of the motor, the motor speed and output torque at the tires may be varied. An electric CVT includes a generator and an electric motor that receives current from the generator. Depending on the current supplied to the motor, the motor speed and output torque may be varied. When using a CVT, the goal is to keep the engine at the most efficient speed while still providing the necessary power to meet the operator's commands. In this situation, the strategy described above of changing only engine fueling in direct response to operator input may work against the efficiency goal. Therefore, an alternative strategy is required to efficiently control operation of a machine utilizing a CVT.
An alternative method of machine control is described in U.S. Patent Publication No. 2008/0103019 of Cronin et al. that published on May 1, 2008 (the '019 publication). Specifically, the '019 publication describes a continuously variable transmission for a machine. The continuously variable transmission includes a driven element, a first operator interface device, a second operator interface device, and a controller. The controller is configured to receive a first displacement signal associated with the first operator interface device, and a second displacement signal associated with the second operator interface device. The controller is further configured to determine a net operator input value as a function of the first and second displacement signals, and to regulate a torque of the driven element in response to the determined net operator input value.
Although the system of the '019 publication may provide efficient regulation of a speed-controlled CVT by separating engine speed from transmission torque control, it may still be less than optimal. In particular, there may be times when engine speed control, in combination with transmission torque control, can further improve machine efficiencies. And the system of the '019 publication may not capture these efficiencies.
The power system of the present disclosure solves one or more of the problems set forth above.